Reducing The Spectral Bandwidth Of Lasers

ABSTRACT

A laser system for semiconductor inspection includes a fiber-based fundamental light source for generating fundamental light that is then converted/mixed by a frequency conversion module to generate UV-DUV laser light. The fundamental light source includes a nonlinear chirp element (e.g., a Bragg grating or an electro-optic modulator) that adds a nonlinear chirp to the seed light laser system prior to amplification by the fiber amplifier(s) (e.g., doped fiber or Raman amplifiers). The nonlinear chirp includes an x 2  or higher nonlinearity and is configured to compensate for the Self Phase Modulation (SPM) characteristics of the fiber-based amplifiers such that fundamental light is generated that has a spectral E95 bandwidth within five times that of the seed light. When multiple series-connected amplifiers are used, either a single nonlinear chirp element is provided before the amplifier string, or chirp elements are included before each amplifier.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application61/670,926, entitled “Reducing The Spectral Bandwidth Of Lasers” filedJul. 12, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to illuminators used inconjunction with inspection systems, such as semiconductor waferinspection systems and photomask inspection systems, and moreparticularly to a fiber amplifier based light source for use with suchinspection systems.

2. Description of the Related Art

FIG. 1(A) is a diagram depicting a simplified UV-DUV laser inspectionsystem 40 utilized in the semiconductor industry for inspecting a targetsample (e.g., a wafer or photomask/reticle) 41. Inspection system 40includes an illumination source 50 that generates laser light L₅₀typically in the UV-DUV range (e.g., a vacuum wavelength less than 350nm), an optical system 42 including one or more objective lenses 43 thatfocus the laser light onto sample 41, a pass-through detector assembly45-1 including a sensor array 46-1 positioned to receive any laser lightthat passes through sample 41 (e.g., for purposes of inspecting aphotomask/reticle), and a reflected light detector assembly 45-2including a sensor array 45-2 positioned to detect any laser light thatis reflected from sample 41 (e.g., for purposes of inspecting wafersurface features). Note that where a wavelength is stated herein withoutqualification, it is assumed to refer to the wavelength in vacuum. Thevacuum wavelength λ=c/ν, where c is the velocity of light in vacuum andν is the frequency in cycles per unit time. The angular frequency, ω, isequal to 2 nν and is in units of radians per unit time. A controller(computing system) 47 controls the operation of the various subsystemsaccording to a software operating program, and processes image datareceived from detector assemblies 45-1 and 45-2 using techniques knownin the art.

It is understood that, in general, shorter wavelength laser lightproduces higher resolution images, which in a laser inspection systemprovides better information regarding features and defects on the imagedsamples. To meet the increasing demand for laser inspection systemshaving ever higher resolution, the current trend in the semiconductorindustry is toward the development of short wavelength UV-DUV laserinspection systems (i.e., systems utilizing laser light below 250 nm).For example, the assignee of the present application is currentlyworking to develop low frequency UV-DUV laser inspections systemsoperating with 213 nm, 206 nm or 193 nm laser light.

A significant obstacle to the development of short wavelength UV-DUVlaser inspection systems is to provide an optical system that caneffectively image the UV laser light. The only two practical materialsavailable for generating the various lenses and elements for the opticalsystem of a UV-DUV laser inspection system (e.g., optical system 42 inFIG. 1(A)) are fused silica and calcium fluoride, with fused silicabeing preferred because calcium fluoride is much more expensive toobtain, polish and mount. Optical systems manufactured from all fusedsilica for use in systems using UV-DUV laser light can only handle alimited bandwidth before the performance degrades beyond acceptablelimits. Specifically, the larger the numerical aperture (NA) and fieldsize, and the shorter the wavelength, the smaller the acceptablebandwidth can be. For example, an all-at refractive objective 266 nmwith 0.8 NA and 1.0 mm field of view may only achieve a bandwidth of 5pm. One approach to deal with a larger bandwidth this is to reduce theglass path by using aspheric surfaces because a single aspheric surfacemay eliminate several equivalent spherical lenses. However the increasedcost and complexity associated with the use of aspheric surface may notbe desirable, and this approach only helps a small amount in most lasersystems.

To minimize the cost and complexity required to generate optical system42 for short wavelength UV-DUV laser inspection system 100, illuminationsource 50 must be able to generate laser light L₅₀ in whichsubstantially all of the light energy is within a narrow bandwidth. Itis typical to specify the bandwidth of a laser light source using a fullwidth half maximum (FWHM) value, which specifies the light's bandwidthrange at one-half of the light's peak power. However, in UV-DUV laserinspection systems, the bandwidth range at which 95% of the energy iscontained (i.e., the light's “E95” bandwidth value) is the moreimportant value. A typical illumination source 41 generates laser lightL₅₀ having a relatively narrow FWHM bandwidth value, but having an E95value that is ten or more times broader than it's FWHM. It is thereforeimportant in laser imaging system 40 to utilize an illumination source50 that generates narrow band UV laser light L₅₀ that is both shortwavelength UV (e.g., laser light having a nominal wavelength value below250 nm) and has a narrow E95 bandwidth (i.e., within ±1%, and preferablywithin ±0.1%, of the nominal or “central” UV frequency).

Narrow band UV light is typically created by generating fundamentallight having a longer wavelength (typically longer than 1 micron), andthen converting the fundamental light using crystals that performnonlinear frequency conversion and frequency mixing to generate UV lighthaving a desired (shorter) wavelength. Because of limitations on thefrequency conversion/mixing process, the fundamental light must have aspecific higher frequency in order to generate UV light at a specifiedshorter wavelength. It is also possible to perform the frequencyconversion/mixing process using other nonlinear processes (e.g., Raman,parametric generation, and four wave mixing (FWM)), but these techniquescan also lead to increased bandwidths and not be suitable for narrowbandwidth optics. Many stages of frequency conversion/mixing aresometimes needed to generate shorter wavelength light having a specifiedfrequency, and power is lost from the light during each frequencyconversation stage. Therefore, in order to generate UV laser light at anacceptable power, it is necessary to generate the fundamental light atsignificantly higher peak power than is needed at the optical system.

There are two types of fundamental light sources used in the generationof narrow band UV light: solid state lasers and fiber lasers.Solid-state lasers can produce laser light having very narrow bandwidthsand high peak power, which allows for the use of less complex (andtherefore lower cost) optical systems, but the wavelength choices forsolid state lasers are very limited and not suitable for some laserinspection systems, and it can be very challenging to obtain reliablehigh power light from a solid state laser. Fiber lasers include anactive gain medium formed by an optical fiber doped with rare-earthelements such as Erbium, Ytterbium, Neodymium, Dysprosium, Holmium,Praseodymium, and Thulium. Fiber lasers are an attractive choice forgenerating fundamental light in laser inspection systems because theygenerate laser light having high peak power, and the frequency of thelaser light can be “tuned” to a specified frequency by altering theamounts of doping materials in the fiber(s). However, as describedbelow, the primary drawback of using fiber lasers to generate high peakpower pulsed fundamental light is Self Phase Modulation (SPM). Ingeneral, SPM is a nonlinear optical effect of light-matter interaction,where ultrashort light pulses travelling in the fiber medium induce avarying refractive index of the medium due to the optical Kerr effect.The variation in refractive index produces a phase shift in the lightpulse, leading to a change of the pulse's frequency spectrum. Thenonlinear SPM effect can dramatically increase the spectral bandwidth ofa fiber laser well beyond the optical requirements of a laser inspectionsystem.

FIG. 1(B) is a diagram showing a conventional fiber-based illuminationsource 50, which is utilized to generate UV laser light L₅₀ ininspection system 40 (shown in FIG. 1(A)). Fiber-based illuminationsource 50 generally includes a fundamental light source 51 forgenerating fundamental light F₅₁ at a specified fundamental frequencyω_(F), and a frequency conversion module 55 that performs the frequencyconversion/mixing process mentioned above in order to generate UV laserlight L₅₀ at a specified UV frequency ω_(UV) that is passed to opticalsystem 42 (see FIG. 1(A)). Fundamental light source 51 includes a seedlaser 52 that generates seed light S₅₂ having the desired fundamentallight frequency ω_(F) at an initial power P₀, a pump laser 53 thatgenerates pump seed light PS at a suitable seed frequency ω_(S), and oneor more fiber amplifiers 54 that utilize the pump seed light PS toamplify seed light S₅₂ in a manner understood in the art, wherebyfundamental light F₅₁ is produced having the desired fundamentalfrequency ω_(F) and an amplified power P_(A) that is substantiallyhigher than initial power P₀. Fundamental light F₅₁ is thenconverted/mixed by frequency conversion module 55 to generate UV laserlight L₅₀ having the desired UV frequency ω_(UV), but at an output powerP_(OUT) that is substantially lower than the amplified power P_(A) offundamental light F₅₁ (i.e., due to energy losses during theconversion/mixing process).

As mentioned above, fundamental light F₅₁ has a bandwidth Δω_(F) that isdetermined in part by the SPM characteristics of fiber amplifier(s) 54,as is well known in the art. SPM gives rise to a phase shift during theamplification process that is intensity dependent given by:

${\varphi_{NL}\left( {L,T} \right)} = {{{U\left( {O,T} \right)}}^{2}\left( \frac{L_{eff}}{L_{NL}} \right)\mspace{14mu} {where}}$$L_{eff} = {\frac{1 - {\exp \left( {{- \alpha}\; L} \right)}}{\alpha}\mspace{14mu} {and}}$$L_{NL} = \frac{1}{\gamma \; P_{0}}$

In the above equations, φ_(NL) is the intensity dependent phase shift, Lis the fiber length, T is time, U is the energy distribution, α is thefiber loss, L_(eff) is the effective length of the fiber consideringfiber loss, L_(NL) is the fiber length at which significant SPM occurs,P₀ is the peak power of the pulse, and γ is the nonlinear coefficient.Because one of the seed light S₅₂ or the pump seed light PS are pulsed,the intensity φ_(NL) changes in time, and this produces a phase thatchanges in time as well. When the phase of light changes in time, thiscreates changes in the wavelength spectrum. The spectral shift δω(T)relative to the central frequency value is given by

${{\delta\omega}(T)} = {{- \frac{\partial\varphi_{NL}}{\partial T}} = {{- \left( \frac{L_{eff}}{L_{NL}} \right)}\frac{\partial}{\partial T}{{U\left( {0,T} \right)}}^{2}}}$

Spectral shift δω(T) is also known as a chirp, or change in theinstantaneous frequency across the pulse.

FIGS. 2(A) and 2(B) are optical spectrum diagrams showing examples of aseed pulse S₅₂ and a fundamental light pulse F₅₁ generated byconventional fundamental light source 51 (shown in FIG. 1(B)), andillustrate the output bandwidth produced by the intensity dependentphase shift generated in conventional fiber-based illumination source 50(FIG. 1(B)). FIG. 2(A) shows that fundamental seed light S₅₂ is aninitial transform limited Gaussian pulse has a peak power ofapproximately 6×10⁶ W centered around a fundamental frequency ω_(F)(e.g., the frequency corresponding to a wavelength of 1030 nm) andhaving an FWHM value of 11 GHz and an E95 energy bandwidth of 23 GHz.FIG. 2(B) shows fundamental light pulse F₅₂, which is produced inamplifier 54 (FIG. 1(B)) using seed light pulse S₅₂ and pump seed lightPS. FIG. 2(B) shows that fundamental light pulse F₅₂ remains generallycentered around fundamental frequency ω_(F) while being amplified to apeak power P_(A) of 10 kW. However, due to the SPM characteristics offiber amplifier(s) 54, the FWHM value of fundamental light pulse F₅₂increases to 222 GHz, and exhibits an E95 energy bandwidth of 286 GHz.This roughly tenfold increase in E95 energy exhibited by fiber-basedfundamental light sources is impractical for generating the type ofnarrow band UV light needed in modern laser inspection systems.

What is needed is a fundamental light source that combines the high peakpower and frequency-tuning capability of fiber-based lasers with thesimplified (low cost) optical systems typically associated withsolid-state lasers.

SUMMARY OF THE INVENTION

The present invention is directed to a fiber-based fundamental lightsource for generating fundamental light F in a laser system in whichseed light and/or partially amplified light is modified to include anonlinear chirp that compensates for the Self Phase Modulation (SPM)characteristics of the fiber-based amplifiers, whereby fundamental lightis generated having both high peak power and a narrow bandwidth.According to a practical embodiment, the laser system also includes afrequency conversion module that converts the fundamental lightgenerated by the fiber-based fundamental light source from a relativelylong fundamental wavelength (e.g., 1030 nm) to generate laser outputlight IL at a desired short UV-DUV wavelength (e.g., 213 nm, 206 nm or193 nm) that is directed to an associated optical system. By using acompensating nonlinear chirp to compensate for SPM prior to completionof the fiber-based amplification process, the present inventionfacilitates the cost-effective manufacture of high resolution laserinspection systems by combining the high peak power and frequency-tuningcapability of fiber-based lasers with the simplified optical systemstypically associated with solid-state lasers.

According to an aspect of the present invention, one or more nonlinearchirp elements (e.g., Bragg gratings, fiber Bragg gratings orelectro-optic modulators) are utilized to generate the compensatingnonlinear chirp, and the nonlinear chirp has a nonlinearity on the orderof x² or higher. In a specific embodiment, the nonlinear chirp U(0,T)has a time-based frequency characterized by the equation

${U\left( {0,T} \right)} = {\exp\left\lbrack {{- \frac{\left( {1 + {\left( {C + {DT} + {ET}^{2} + {FT}^{3} + {GT}^{4} + \ldots}\mspace{14mu} \right)}} \right)}{2}}\frac{T^{2}}{T_{0}^{2}}} \right\rbrack}$

where T is time, i indicates the imaginary part of the amplitude thatcontains the phase term, and wherein at least one of one E, F and G isnon-zero. The present inventor has determined that a nonlinear chirphaving a nonlinearity of x² or higher is required to achieve fundamentallight having a narrow spectral E95 bandwidth that is in a range definedby five times the (initial) spectral E95 bandwidth of the seed lightgenerated by the seed laser (e.g., in the range of 1 and 100 GHz). Byway of comparison, a linear chirp generated in a manner similar to thatused in the present invention (e.g., using a Bragg grating configured togenerate a linear chirp) is capable of generating a FWHM value that isclose to that of the seed light, but its E95 bandwidth is over ten timeshigher. Accordingly, the SPM characteristics generated by a fiber-basedamplifier (e.g., either a doped fiber amplifier or a fiber Ramanamplifier) require an x² or higher nonlinearity (e.g., that at least oneof E, F and G in the above equation must be non-zero) to adequatelycompensate for SPM characteristics to the degree required by highresolution laser inspection systems. According to an embodiment, asingle nonlinear chirp element is positioned in the laser optical pathbetween the seed laser and a series of fiber amplifiers, wherein thenonlinear chirp element is configured to generate a single nonlinearchirp that compensates for the cumulative SPM generated by all of theseries-connected fiber amplifiers. A benefit of this single elementapproach is that the seed/amplified light encounters a minimum number ofnonlinear chirp element, thereby minimizing power loss. According to analternative embodiment, a nonlinear chirp element is positioned in thelaser light optical path in front of each of the series-connected fiberamplifier, wherein each nonlinear chirp element is configured togenerate a component nonlinear chirp that compensates for individual SPMcharacteristics associated with the subsequent fiber amplifier. Abenefit of this multiple-element approach is that it simplifies thecompensating nonlinear chirp calculation by addressing the SPMcharacteristics of each fiber amplifier (i.e., instead of having tocalculate a compensating nonlinear chirp for the cumulative SPM ofmultiple series-connected amplifiers).

According to another embodiment of the present invention, a Ramanamplifier receives both continuous wave (CW) seed light and pulsed pumpseed light, and the nonlinear chirp NLC is added to the CW seed light bya phase modulator. Subsequent “downstream” series-connected Ramanamplifiers receive pulsed pump seed light, but not CW seed light. Abenefit of utilizing this Raman amplifier approach is the Raman spectralshift allows the laser to operate at high power levels at wavelengthsoutside the range that is possible with standard fiber lasers.

These and other advantages of the present invention will become apparentto those skilled in the art from the following detailed description ofthe invention and the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings in which:

FIG. 1(A) is a simplified block diagram illustrating a laser inspectionsystem;

FIG. 1(B) is a simplified block diagram illustrating a conventionalpulsed fiber laser;

FIG. 2(A) is an output spectrum diagram showing a seed pulse generatedby the seed laser in the pulsed fiber laser of FIG. 1(B);

FIG. 2(B) is an output spectrum diagram showing amplified light andillustrating the effect of Self Phase Modulation on the spectralbandwidth of a fiber laser;

FIG. 3 is a simplified block diagram showing a laser system according toa generalized embodiment of the present invention;

FIG. 4 is an output spectrum diagram showing amplified fundamental lightgenerated by the laser system of FIG. 3;

FIG. 5 is an output spectrum diagram showing amplified fundamental lightgenerated using a linear chirp;

FIG. 6 is a simplified block diagram showing a laser system according toan embodiment of the present invention;

FIG. 7 is a simplified block diagram showing a laser system according toanother embodiment of the present invention;

FIG. 8 is a simplified block diagram showing a laser system according toyet another embodiment of the present invention; and

FIGS. 9(A) and 9(B) are output spectrum diagrams showing light generatedby the laser system of FIG. 8.

DETAILED DESCRIPTION

The present invention relates to an improvement in laser technology. Thefollowing description is presented to enable one of ordinary skill inthe art to make and use the invention as provided in the context of aparticular application and its requirements. As used herein, directionalterms such as “higher”, “lower”, “front” and “downstream” are intendedto provide relative positions for purposes of description, and are notintended to designate an absolute frame of reference. Variousmodifications to the preferred embodiment will be apparent to those withskill in the art, and the general principles defined herein may beapplied to other embodiments. Therefore, the present invention is notintended to be limited the particular embodiments shown and described,but is to be accorded the widest scope consistent with the principlesand novel features herein disclosed.

FIG. 3 shows a laser system 90 according to a generalized embodiment ofthe present invention. Laser system 90 includes a fiber-basedfundamental laser system 100 for generating fundamental light F at anominal fundamental frequency ω_(F), and a frequency conversion module94 including various elements for converting/mixing fundamental light Fto generate laser output light L at a desired nominal output frequencyω_(OUT), which is then passed to an associated optical system (notshown).

According to an embodiment, laser system 90 is utilized as theillumination source in a UV-DUV laser inspection system in the mannerdescribed above with reference to FIG. 1(A)). Referring to the rightside of FIG. 3, in a specific embodiment, fundamental light F at anominal fundamental frequency ω_(F) that corresponds to a wavelength ofapproximately 1 μm (e.g., 1030 nm), and frequency conversion module 94is configured to convert fundamental light F to a specified outputfrequency ω_(OUT) that is more than 2.5 times that of nominalfundamental frequency (i.e., such that output frequency ω_(OUT) has acorresponding wavelength that is less than 40% of that of nominalfundamental frequency ω_(F); e.g., a wavelength below 400 nm, and moreparticularly such that laser light L has a nominal wavelength of 355 nm,266 nm, 213 nm, 206 nm or 193 nm). Laser light L also has an outputbandwidth Δω_(OUT) that is proportional to a fundamental spectralbandwidth Δω_(F) of fundamental light L. A frequency-conversionarchitecture capable of performing the function of frequency conversionmodule 94 is disclosed in co-owned and co-pending U.S. patentapplication Ser. No. 13/558,318, entitled “Solid-State Laser AndInspection System Using 193 nm Laser”, which is incorporated herein byreference in its entirety. Although the present invention is describedherein with specific reference to UV-DUV laser inspection system, thepresent invention is believed to be useful in other laser systems aswell.

Referring the left side of FIG. 3, similar to conventional fiber-basedlasers, fundamental light source 100 includes a seed laser 105, afiber-based amplifier 130, and a pump seed laser 140. Seed laser 105(e.g., a pulsed light oscillator, a solid state laser, a diode laser, again-switched laser diode, or a quantum dot laser) generates seed lightS₀ having an initial (relatively low) peak power P₀ and an initialspectral bandwidth Δω_(S), and transmits seed light S₀ along an opticalpath O using known techniques. Amplifier 130 is disposed in optical pathO and including a fiber-based amplification mechanism (e.g., a dopedfiber amplifier or a fiber Raman amplifier) that amplifies the seedlight S₀ such that fundamental (amplified) light F output from amplifier130 has a fundamental peak power P₁ that is substantially higher (e.g.,ten times) than initial peak power P₀ of seed light S₀. Pump seed laser140 (e.g., a pulsed light oscillator, a solid state laser, a diodelaser, a gain-switched laser diode, or a quantum dot laser) generatesand transmits pump seed light PS to amplifier 130 according to knowntechniques.

According to an aspect of the present invention, fundamental lightsource 100 includes a nonlinear chirp element 150 disposed in opticalpath O (e.g., between seed laser 105 and amplifier 130) that combinesseed light S₀ (or, as explained below, partially amplified seed light)with a nonlinear “pre-chirp” (chirp) NLC that compensates for theinherent SPM characteristics of amplifier 130. In one embodiment,nonlinear chirp element 150 is implemented using a Bragg grating, afiber Bragg grating or an electro-optic modulator that is configured togenerate nonlinear chirp NLC in a manner that “mirrors” the amplifier'sSPM characteristics (i.e., such that the frequency bandwidth andfrequency amplitudes of the nonlinear pre-chirp NLC are substantiallyopposite to those of the amplifier's SPM characteristics, whereby thenonlinear chirp NLC compensates for (i.e., effectively cancels orgreatly reduces) the amplifier's SPM characteristics such thatfundamental spectral bandwidth Δω_(F) of fundamental light F is in arange defined by five times said initial spectral E95 bandwidth Δω_(S)of seed light S₀.

According to another aspect of the present invention, chirp element 150is constructed to generate nonlinear pre-chirp with a nonlinearity onthe order of x² or higher. Specifically, one or more nonlinear chirpelements 150 constructed to generate a nonlinear pre-chirp U(0,T) havingtime-based frequency characterized by the equation

${U\left( {0,T} \right)} = {\exp\left\lbrack {{- \frac{\left( {1 + {\left( {C + {DT} + {ET}^{2} + {FT}^{3} + {GT}^{4} + \ldots}\mspace{14mu} \right)}} \right)}{2}}\frac{T^{2}}{T_{0}^{2}}} \right\rbrack}$

where T is time, i indicates the imaginary part the amplitude thatcontains the phase term, and wherein at least one of one of E, F and Gis non-zero. That is, the equation utilized to generate nonlinear chirpelement 150 has at least one time-based component having an order of T²or higher. Accordingly, nonlinear chirp element 150 is constructed byfirst measuring and quantifying the SPM characteristics of amplifier 130using known techniques, then values for C, D, E, F and G are calculatedthat compensate for (mirror) the measured/quantified characteristics,and then a nonlinear chirp element 150 is constructed to include therequisite compensating non-linear pre-chirp (e.g., a Bragg grating isconstructed having a periodic refractive index variation that implementsthe nonlinearity defined by the calculated values).

FIG. 4 shows an exemplary output spectrum diagram for fundamental lightF generated from seed pulse using a nonlinear pre-chirp according to thepresent invention. For comparison purposes, the seed pulse utilized inthis example is the same as that shown in FIG. 2(A) and described above.Note that fundamental light F is a 10 kW peak power laser pulse having afundamental spectral bandwidth Δω_(F) around a nominal fundamentalfrequency ω_(F) (e.g., 1030 nm), where fundamental spectral bandwidthΔω_(F) is characterized by having a FWHM value of 21 GHz (compared tothe 11 GHz FWHM value of the seed pulse; see FIG. 2(A)) and a spectralE95 bandwidth of 41 GHz (compared to 23 GHz E95 value of the seed pulseshown in FIG. 2(A)). The substantial improvement provided by adding anonlinear chirp is clearly indicated by comparing FIG. 4 with FIG.2(A)—the nonlinear chirp produced a ten-times reduction in FWHM (i.e.,from 222 GHz to 21 GHz) and an approximately seven-times improvement inspectral E95 bandwidth (i.e., from 286 GHz to 41 GHz).

For comparison purposes, FIG. 5 shows an output spectrum diagram forfundamental light F_(LINEAR) generated using a linear pre-chirp andhaving a 10 KW peak power pulse. That is, instead of using a pre-chirphaving a nonlinearity of ×2 or higher, FIG. 5 illustrates the effect ofmodifying the seed pulse of FIG. 2(A) using a pre-chirp constructedusing the following linear equation (function):

${U\left( {0,T} \right)} = {\exp\left\lbrack {{- \frac{\left( {1 + {\; C}} \right)}{2}}\frac{T^{2}}{T_{0}^{2}}} \right\rbrack}$

Similar to the nonlinear example, a linear pre-chirp element is matchedusing the above equation to the fiber characteristics and the amount ofSPM generated by the amplifier so that the inherent SPM chirp iscompensated as much as possible. As indicated in FIG. 5, fundamentallight F_(LINEAR) exhibits a central spike with a relatively narrow FWHMvalue similar to that generated by the nonlinear chirp (i.e., 23 GHz).However, as can be seen in the lower portion of FIG. 5, the spectralwidth of the laser pulse at the E95 energy point is extremely broad(i.e., 300 GHz or more). This large E95 value is present because thereis still significant inherent chirp remaining on the pulse that is notlinear. As such, it is not believed possible to utilize a linearpre-chirp that sufficiently compensates for the nonlinear SPMcharacteristics of a fiber-based amplifier to produce the high peakpower, narrow bandwidth laser light required for high resolution UV-DUVlaser inspection systems.

Referring again to FIG. 3, fundamental laser system 100 is illustratedwith a single nonlinear chirp element 150 disposed in optical path Obetween seed laser 105 and a single amplifier 130. With thisarrangement, modified seed light S₁ exiting nonlinear chirp element 150has the same nominal wavelength ω_(S), similar peak power P₀′ (i.e.,peak power P₀′ substantially equal to but slightly lower than initialpeak power P₀ of seed light S₀), and a modified spectral bandwidth Δω₁that is different from said initial narrow spectral bandwidth Δω_(S) inthat it includes nonlinear chirp NLC. Fiber amplifier 130 includes afiber-based amplification mechanism that amplifies modified seed lightS₁ such that said fundamental light F has the required UV-DUVfundamental spectral bandwidth Δω_(F). In some embodiments (e.g., in thecase where fiber amplifier 130 is a doped fiber amplifier), the nominalfundamental frequency ω_(F) of fundamental light F is substantiallyidentical to the initial (seed) frequency ω_(S). In other embodiments(e.g., in the case where fiber amplifier 130 is a Raman amplifier), thenominal fundamental frequency ω_(F) of fundamental light F issubstantially different from initial (seed) frequency ω_(S).

Although the invention is described above with reference to a systemincluding a single fiber-based amplifier 130 for brevity, it isunderstood that most high peak power fiber-based laser systems utilizetwo or more series-connected amplifiers to achieve a sufficiently highfundamental light so that the output laser light has an adequate powerlevel. FIG. 6 shows a portion of a laser system 90A according to anembodiment of the present invention in which a fiber-based fundamentallaser system 100A includes a single seed laser 105A, an amplifiersection 130A including multiple series-connected amplifiers 130A-1,130A-2 . . . 130A-n, and a single nonlinear chirp element 150A that isdisposed in the optical path between seed laser 105A and amplifiersection 130A. In this case, single nonlinear chirp element 150A isconfigured to generate a single nonlinear pre-chirp NLC that is added tomodified seed light S₁ to compensate for the cumulative SPM effectgenerated by all of series-connected fiber amplifiers 130A-1, 130A-2 . .. 30A-n. That is, the cumulative SPM characteristics of series-connectedamplifiers 130A-1, 130A-2 . . . 130A-n is determined and quantified, andthen nonlinear chirp element 150A is constructed to generate a nonlinearpre-chirp that compensates for the cumulative SPM characteristics. Abenefit of using single nonlinear chirp element 150A is that thisarrangement minimizes power loss to the modified seed light passed toamplifier section 130A.

FIG. 7 shows a portion of a laser system 90B according to anotherembodiment of the present invention in which a fiber-based fundamentallaser system 100B includes seed laser 105B, multiple amplifiers 130B-1,130B-2 . . . 130B-n, and multiple nonlinear chirp elements 150B-1,150B-2 . . . , where at least one nonlinear chirp element is disposed inthe optical path between two amplifiers (e.g., element 150B-2 isdisposed between amplifiers 130B-1 and 130B-2). In this case, eachnonlinear chirp element 150B-1, 150B-2 . . . is configured to generate anonlinear pre-chirp that is added either to the seed light or topartially amplified seed light (e.g., nonlinear pre-chirp NLC2 is addedby element 150B-2 to partially amplified light S_(B2) to generateamplified light S_(B3)) to compensate for the individual SPM effectgenerated by an associated fiber amplifier 130B-1, 130B-2 . . . 130B-n.For example, nonlinear pre-chirp NLC1 compensates for the SPMcharacteristics of amplifier 130B-1, and is added by element 150B-1 toseed light S₀ to generate modified seed light S_(B1). Similarly,nonlinear pre-chirp NLC2 compensates for the SPM characteristics ofamplifier 130B-2, and is added by element 150B-2 to partially amplifiedlight S_(B2) to generate amplified light S_(B3). The cumulative effectof nonlinear chirp elements 150B-1, 150B-2 . . . is similar to that ofthe single chirp element embodiment, but the multiple-element approachsimplifies the compensating nonlinear chirp calculation by addressingthe SPM characteristics of each fiber amplifier individually (i.e.,instead of having to calculate a compensating nonlinear chirp for thecumulative SPM of multiple series-connected amplifiers).

FIG. 8 shows a portion of a laser system 90C according to anotherembodiment of the present invention in which a fiber-based fundamentallaser system 100B utilizes series-connected Raman amplifiers 130C-1,130C-2 . . . 130C-n. In this case, seed laser 105C-1 is implemented by acontinuous wave (CW) laser (e.g., one of a solid state laser, a diodelaser and a quantum dot laser), and the nonlinear chirp element isimplemented by a phase modulator 150C disposed in the optical pathbetween (first) Raman amplifier 130C-1 and CW seed laser 105C such thatRaman amplifier 130C-1 receives both CW seed light S₀₁ and a nonlinearchirp NLC from phase modulator 150C. Fundamental laser system 100B alsoincludes multiple pulse-type pump seed lasers 105C-21, 105C-22 . . .105C-2 n (e.g., gain-switched laser diodes or pulsed light oscillators)that respectively supply pulse-type pump seed light to respective Ramanamplifiers 130C-1, 130C-2 . . . 130C-n. For example, pulse-type pumpseed laser 105C-21 supplies pulse-type pump seed light S₀₂₁ to Ramanamplifier 130C-1, and pulse-type pump seed laser 105C-22 suppliespulse-type pump seed light S₀₂₂ to Raman amplifier 130C-2. FIGS. 9(A)and 9(B) show results of using a nonlinear chirp to compensate for Crossphase modulation (XPM) for a Raman amplifier in accordance with theembodiment shown in FIG. 8. XPM is 2× worse than SPM obtained instandard fiber lasers for the same peak powers. A benefit of utilizingthis Raman amplifier approach is that high powers at wavelengths beyondthose that are available with standard fiber lasers are more practical.One or more Raman shifts can be used to increase the wavelength.

Although the present invention has been described with respect tocertain specific embodiments, it will be clear to those skilled in theart that the inventive features of the present invention are applicableto other embodiments as well, all of which are intended to fall withinthe scope of the present invention.

1. A laser system for generating laser output light having a nominalUV-DUV output frequency, the laser system comprising: a fiber-basedfundamental light source for generating fundamental light at a nominalfundamental frequency including: one or more seed lasers including meansfor generating seed light having an initial peak power and an initialspectral bandwidth, and for transmitting the seed light along an opticalpath; one or more amplifiers disposed in the optical path and includingmeans for amplifying the seed light generated by said one or more seedlasers such that amplified light output from the one or more amplifiershas a fundamental peak power that is substantially higher than theinitial peak power; and one or more nonlinear chirp elements disposed inthe optical path for combining one of the seed light and the amplifiedlight with one or more nonlinear chirps; and a frequency conversionmodule including means for converting said fundamental light from saidnominal fundamental frequency to said laser output light L having saidnominal output frequency, wherein the one or more amplifiers haveinherent Self Phase Modulation (SPM) characteristics, and wherein thenonlinear chirp compensates for said SPM characteristics of the one ormore amplifiers such that said fundamental light generated by said lasersystem has a fundamental spectral bandwidth that is in a range definedby five times said initial spectral bandwidth of said seed light.
 2. Thelaser system of claim 1, wherein said frequency conversion moduleincluding means for generating laser output light such that said outputfrequency is more than 2.5 times said nominal fundamental frequency. 3.The laser system of claim 2, wherein said one or more seed lasers andsaid one or more amplifiers comprise means for generating saidfundamental light such that said nominal fundamental frequency is afrequency having a corresponding wavelength of approximately 1 μm, andwherein said frequency conversion module including means for generatingsaid laser output light such that said output frequency is a frequencyhaving a corresponding wavelength in a range of 190 nm and 400 nm. 4.The laser system of claim 1, wherein each of said one or more seedlasers comprises one of a pulsed light oscillator, a solid state laser,a diode laser, a gain-switched laser diode, and a quantum dot laser. 5.The laser system of claim 1, wherein said one or more amplifierscomprise one of a doped fiber amplifier and a Raman amplifier.
 6. Thelaser system of claim 1, further comprising a pump seed laser fortransmitting pump seed light to said one or more amplifiers.
 7. Thelaser system of claim 1, wherein each of said one or more nonlinearchirp elements comprises one of a Bragg grating, a fiber Bragg gratingand an electro-optic modulator.
 8. The laser system of claim 7, whereinsaid one or more nonlinear chirp elements comprise means for generatingsaid nonlinear pre-chirp with a nonlinearity on the order of x² orhigher.
 9. The laser system of claim 7, wherein said one or morenonlinear chirp elements comprise means for generating said nonlinearpre-chirp U(0,T) having time-based frequency characterized by theequation${U\left( {0,T} \right)} = {\exp\left\lbrack {{- \frac{\left( {1 + {\left( {C + {DT} + {ET}^{2} + {FT}^{3} + {GT}^{4} + \ldots}\mspace{14mu} \right)}} \right)}{2}}\frac{T^{2}}{T_{0}^{2}}} \right\rbrack}$where T is time and i is associated with amplitude, and wherein at leastone of one of E, F and G is non-zero.
 10. The laser system of claim 1,wherein one or more nonlinear chirp elements are disposed between theone or more seed lasers and the one or more amplifiers in the opticalpath such that modified seed light exiting the one or more nonlinearchirp elements has a modified power that is substantially equal to saidinitial peak power and a modified spectral bandwidth, that is differentfrom said initial spectral bandwidth, and wherein said one or moreamplifiers includes means for amplifying the modified seed light suchthat said nominal fundamental frequency of said fundamental light is inthe UV-DUV range.
 11. The laser system of claim 10, wherein said one ormore amplifiers comprises a plurality of amplifiers connected in seriesin said optical path.
 12. The laser system of claim 1, wherein said oneor more amplifiers comprises a plurality of amplifiers including a firstamplifier and a second amplifier, and wherein said one or more nonlinearchirp elements comprise at least one nonlinear chirp elements disposedbetween said first and second amplifiers in said optical path.
 13. Thelaser system of claim 1, wherein said one or more seed lasers comprisesa continuous wave (CW) laser for generating CW seed light and a pulsedlaser for generating pulsed pump seed light, wherein said one or moreamplifiers comprises a Raman amplifier disposed to receive said CW seedlight and said pulsed pump seed light, wherein said one or morenonlinear chirp elements comprises a phase modulator disposed in theoptical path between the Raman amplifier and the CW laser such that saidRaman amplifier receives both said CW seed light and said nonlinearchirp from said phase modulator.
 14. The laser system of claim 13,wherein said CW laser comprises one of a solid state laser, a diodelaser and a quantum dot laser, and wherein said pulsed laser comprisesone of a gain-switched laser diode and a pulsed light oscillator. 15.The laser system of claim 13, wherein said said one or more amplifierscomprises a second Raman amplifier disposed in the optical path toreceive amplified light output from said Raman amplifier, and whereinsaid one or more seed lasers further comprises a second pulsed laserdisposed to transmit second pulsed pump seed light into said secondRaman amplifier.
 16. A fiber-based fundamental light source forgenerating fundamental light at a nominal fundamental frequency a lasersystem, the fiber-based fundamental light source comprising: one or moreseed lasers including means for generating seed light having an initialpeak power and an initial spectral bandwidth, and for transmitting theseed light along an optical path; one or more amplifiers disposed in theoptical path and including means for amplifying the seed light generatedby said one or more seed lasers such that amplified light output fromthe one or more amplifiers has a fundamental peak power that issubstantially higher than the initial peak power; and one or morenonlinear chirp elements disposed in the optical path and includingmeans for modifying one of the seed light and the amplified light toinclude one or more nonlinear chirps that compensate for Self PhaseModulation (SPM) characteristics of the one or more amplifiers.
 17. Thefiber-based fundamental light source of claim 16, wherein each of saidone or more nonlinear chirp elements comprises one of a Bragg grating, afiber Bragg grating and an electro-optic modulator.
 18. The fiber-basedfundamental light source of claim 17, wherein said one or more nonlinearchirp elements comprise means for generating said nonlinear pre-chirpwith a nonlinearity on the order of x² or higher.
 19. The fiber-basedfundamental light source of claim 18, wherein said one or more nonlinearchirp elements comprise means for generating said nonlinear pre-chirpU(0,T) having time-based frequency characterized by the equation${U\left( {0,T} \right)} = {\exp\left\lbrack {{- \frac{\left( {1 + {\left( {C + {DT} + {ET}^{2} + {FT}^{3} + {GT}^{4} + \ldots}\mspace{14mu} \right)}} \right)}{2}}\frac{T^{2}}{T_{0}^{2}}} \right\rbrack}$where T is time, i is associated with amplitude, and wherein at leastone of one of E, F and G is non-zero.
 20. A method for generatingfundamental light using a fiber-based laser system, the methodcomprising: generating seed light having an initial spectral bandwidth;amplifying the seed light using one or more fiber-based amplifiers toproduce amplified light having a peak power that is substantially higherthan the initial peak power; and modifying one of the seed light and theamplified light to include a nonlinear chirp, wherein the nonlinearchirp has a nonlinearity on the order of x² or higher and substantiallyentirely compensates for Self Phase Modulation (SPM) characteristics ofsaid one or more fiber-based amplifiers such that said fundamental lighthas a fundamental spectral bandwidth that is within a range defined byfive times said initial spectral bandwidth.